I just put mine in the trunk. I used the Optima Red Top and the Detroit Speed tray machined for the Optima.
Mounted to the trunk hinge plate are two fuses. One is a giant from a stereo shop capable of 100, 150 or 300 amp fusing. The second is smaller and runs to the stereo amp. I wanted to protect the car from dropping the battery to ground if something chaffed the wire in the car.
The two guage wire runs up through the wire trough to the top of the passenger toe pan. At the very top, as it meets the firewall, there's a flat spot. There I cut out a hole for the pass through connection. I used the insulated connector found in Jegs, (about 9 -13 bucks.)
I removed the windshield washer bottle and found the bracket left behind was perfect for a Ford type solenoid. Installed the solenoid, ran cable form the toe pan to the solenoid, then to the starter. Added a line from the hot side of the solenoid over to the connection on the radiator brace.
Because of the headers, I re-introduced heat soak as a problem to my car. The solenoid and batery in the trunk fixes this cleanly. The one thing I'm going to change is the location of the solenoid. I'm going to mount it four inches past the massive fuse in the trunk. I think this will be the perfect resolution and design.
It didn't turn out to be cheap by any means. Here's some numbers...
140 Battery tray
72 18 feet 2 gauge
20 10 feet 4 gauge
20 20 feet 8 gauge,(17 ft too much.)
47 Fuse holder - starter
24 Two 150 amp fuses, (one for back-up)
12 Fuse and fuse holder,(8 guage to rad.)
25 Mr. Gasket Ford solenoid kit.
(Could have been done cheaper)
120 Assorted solid brass connectors
Show quality, auto stereo shop type.
This might sound nuts but the only item I paid too much for is the solenoid kit. This could have been done just by buying the solenoid alone and making your own jumper for the stock starter solenoid.
Deb was with me at every single step and was blown away at the cost just to move the battery to the trunk.
PS: I forgot to add the fuse for the amp and the insulated connector. Another thirty bucks. This hobby, this passion, is not cheap.